Legal regulations prescribe the monitoring of the composition of the exhaust gas of internal combustion engines for compliance with limiting values. For that purpose, undesirable substances in the exhaust gas such as nitrogen oxides and carbon monoxide are converted into substances considered to be non-critical such as water vapor, carbon dioxide and nitrogen with the aid of regulated three-way catalytic converters. This conversion requires that the air-fuel mixture supplied to the internal combustion engine is within a certain range of composition around a stoichiometric composition. This composition is referred to as the parameter lambda=1. The composition of the air-fuel mixture is monitored using exhaust gas sensors provided in the exhaust gas tract of the internal combustion engine, for example, in the form of broadband lambda sensors, which determine the oxygen partial pressure. Broadband lambda sensors are made up of among other things, a Nernst cell, which determines the concentration of oxygen, a pump cell which adjusts the oxygen concentration, a cavity by which the two cells are connected, and a diffusion barrier through which the exhaust gas is able to diffuse from the exhaust gas tract into the cavity. In an alternative embodiment, the oxygen concentration is determined using a discrete-level sensor, also referred to as a two-point lambda sensor, the signal of which indicates an abrupt change in the output signal in a narrow range around lambda=1.
In both cases, the lambda sensor is based on a solid electrolyte, which is conductive for oxygen ions at a temperature above 350° C., which is referred to as the activation temperature. The working temperature of the exhaust gas sensor, also referred to as the nominal temperature, is typically between 650° C. and 850° C. The temperature at which the lambda sensor is operationally ready and meets the requirements in an engine control system lies between the activation temperature and the nominal temperature of the sensor. Above this temperature, the lambda regulation may be activated and contribute to reducing the emission of undesirable components in the exhaust gas of the internal combustion engine. For the purpose of reducing the exhaust gas emissions, the lambda sensor must therefore, on the one hand, reach a suitable temperature as quickly as possible, and on the other hand, detect operational readiness as quickly as possible. This finding is based on a measurement of the temperature of the lambda sensor.
Discrete-level sensors may be designed in such a way that a voltage on a solid electrolyte is picked up, one side being exposed to an exhaust gas of an internal combustion engine and the other side being exposed to the outside air as the reference gas. Such a system is known from WO2009/156007, in which a pumped oxygen reference is used as the reference gas. WO2009/156007 describes a lambda sensor for measuring the exhaust gas lambda in an exhaust area of an internal combustion engine, which contains a first electrode 20 situated in a measuring gas cavity 18 connected to the exhaust gas, the lambda sensor containing a second electrode 24, which is connected to the first electrode 20 via a solid electrolyte 22 conducting oxygen ions and which is situated in a reference gas channel 26, characterized in that a specifically formed oxygen storage 40, 50, 62, 70 is provided in reference gas channel 28. Such a discrete-level sensor is sold by the manufacturer Bosch under the name “LSF Xfour.”
The influence of the temperature of the lambda sensor on its output signal and on its accuracy in determining the composition of the air-fuel mixture is known. From DE 102008005110A1 a method is known for operating at least one lambda sensor in an exhaust gas system of an internal combustion engine having a lambda regulating system for regulating an air/fuel mixture ratio of a combustion process of an internal combustion engine, the exhaust gas system having at least one heating element for heating up the lambda sensor, which is heated in one method step, and the heating element being heated in a regulated way by a heating element control, characterized in that                in a first group of steps at least two of the following parameters of the lambda sensor are used and/or detected;applying a defined or undefined heating power;            detecting the ohmic resistance of the heating element of the lambda sensor;    detecting the ohmic resistance of the signal electrodes of the lambda sensor;    detecting the electrical sensor signal of the lambda sensor;            in a second group of steps, from at least one detected parameter value, a change of the detected parameter is ascertained or detected,        in a third group of steps a comparison is made in each case of the ascertained change of the detected parameter with a predefined reference value for the expected correlating change of the other used and/or detected parameter(s), and the process is advanced to a fourth group of steps using the result of the comparison, and        in a fourth group of steps, a correction value is determined, at least one operational reference value being obtained from a reference value supply device and being supplemented to form an operational setpoint value of the lambda sensor using the determined correction value.        
The aim of the method and the associated control is the correction of the output signal of the lambda sensor with respect to the influences of manufacturing-related variations or aging and, among other things, the temperature characteristic of the internal resistance of the lambda sensor. The publication does not deal with an extension of the measuring range of the lambda sensor to lower operating temperatures.